Multi-focal type optical lenses having a plurality of focal points are known from the past, and have been applied to various optical systems such as the human eye optical system, camera optical systems, or the like. For example, with contact lenses used as corrective optical elements for refractive error, alternative optical elements after lens extraction or the like with the optical system of the human eye, or with intraocular lenses used for insertion in the human eye, by applying multi-focal lenses, it is possible to compensate for the decrease or loss of accommodation function of eye in the human body.
Particularly in recent years, there is an increase in people continuing to use contact lenses even when they reach the age of having presbyopia. People with presbyopia have a decrease in focus accommodation function, so a symptom appears of having difficulty focusing on nearby items. Thus, multi-focal contact lenses which can also focus on nearby items become necessary for presbyopia patients. Also, for patients who have undergone cataract surgery, the lens which is in charge of the adjustment function is removed, so even if an intraocular lens is inserted as a replacement, the symptom of difficulty seeing close up remains. A multi-focal function that offers a plurality of focal points is necessary for that intraocular lens as well. Thus, there is a great increase in the need for multi-focal lenses reflecting the aging society of recent years.
However, as a method for realizing this multi-focal lens, examples are known of a refraction type multi-focal lens for which a plurality of focal points are formed based on the principle of refraction, and of a diffractive type multi-focal lens for which a plurality of focal points are formed based on the principle of diffraction. With the latter diffractive type multi-focal lens, equipped are a plurality of diffractive structures formed in concentric circle formed on the optical part of the lens, and a plurality of focal points are given by the mutual interference effect of light waves that passed through the plurality of diffractive structures (zones). Thus, compared to the refraction type lens with which a focal point is given by the refraction effect of light waves at a refracting surface comprising boundary surfaces with different refractive indexes, with the diffractive type multi-focal lens, there are advantages such as being able to set a high lens power while inhibiting an increase in lens thickness.
Typically, the diffractive type multi-focal lens has a diffractive structure by which the diffractive zone pitch gradually becomes smaller as it goes from the lens center toward the periphery according to a rule called the Fresnel zone, and this has multiple focal points by using different orders of diffracted light generated from that structure. When using a diffractive multi-focal lens as a contact lens or an intraocular lens, normally, 0th order diffracted light is the focal point for far vision, and +1 order diffracted light is the focal point for near vision. By distribution of this diffracted light, it is possible to make a bifocal lens having focal points for far and near vision. The general Fresnel zone constitution is basically the zone pitches having the zone outer diameter radius determined by Equation 1 below. This Equation 1 is hereafter called a Fresnel zone setting equation.
                              r          n                =                              nK            P                                              [                  Equation          ⁢                                          ⁢          1                ]            
rn is the outer diameter radius of the nth zone obtained from Equation 1. K is a constant. P is addition power for setting the focus point of first order diffracted light with the focus point of 0th order diffracted light as a reference, and by varying this, it is possible to change the focal point position of the first order diffracted light.
For example when the focal point by 0th order diffracted light is a focal point for far vision, and first order diffracted light is set as the focal point for near vision, when P (the addition power noted above) is made larger, the focal point position for near vision moves closer to the lens. Specifically, when using that lens for the human eye, items that are closer become visible. Conversely, when P is made smaller, the focal position for near vision recedes away from the lens. In this case, when the lens is used in the human eye, the near points that are visible recede away.
For patients with advanced presbyopia, or patients who have an intraocular lens inserted, power of accommodation of the crystalline lens decreases or is lost, so it is preferable to use a lens for which the focal point is matched in the nearer direction as with the former example. In other words, an item is needed for which the addition power is set to be large. On the other hand, for patients for which the power of accommodation has not decreased that much, even if the near focal point position is not made that near, it is possible to see near objects by joint use with one's own residual power of accommodation, so there are cases when large addition power does not need to be set. Taking into consideration the status of the eyes of these patients, it is possible to obtain bifocal lenses that can be suitably used at different required powers for each patient by setting P.
However, with this bifocal type lens, we found that the problems to be resolved noted below remain.
With bifocal lenses, there are two focal points existing together, the far focal point and near focal point, and between these points, there is a blank region in which no focal point exists. The larger the addition power is made, the more this blank region expands. For patients with decreased power of accommodation, a diffractive multi-focal lens having large addition power is suitable, but when using that lens, there is the problem that though both far and near objects are visible, when objects between those focal points are viewed, they cannot be seen clearly.
Power of accommodation decreases as age increases. The power of accommodation is typically defined using numerical values expressed in diopter units in the opthalmological field, and the larger the numerical value, the greater the power of accommodation. The residual power of accommodation differs for each person, but a typical trend is said to be for the residual power of accommodation to decrease to approximately 2 to 3 diopters in the mid forties, approximately 1.5 diopters in the fifties, and less than 1 diopter in the sixties. Hereafter, diopters are noted as D as the refractive power unit.
Normally, when viewing an object 30 cm in front of oneself, the power of accommodation of the human eye that is required is approximately 3.3 D, and for example when a person in his fifties views an object at that point, the power of accommodation is insufficient by approximately 1.8 to 2 D. When such a patient uses bifocal lenses, addition power of approximately 1.8 to 2 D is needed. Also, for patients with an intraocular lens inserted, the crystalline lens as been removed, so there is almost no residual power of accommodation. With such patients, addition power of 3 to 3.5 D is necessary. When setting the addition power for the lens with an intraocular lens as a multi-focal lens, it is necessary to further change the addition power given to the lens by the set position of the intraocular lens within the eye, and for eyes with an intraocular lens inserted, to give the addition power of 3 to 3.5 D noted above, it is necessary to give addition power of 3.5 to 4 D to the lens itself.
When a patient with advanced presbyopia or a patient with an intraocular lens inserted uses a bifocal lens set so as to have this addition power, there is a new problem of having it be difficult to see the intermediate region between the far and near range. A multi-focal lens is needed that is capable of generating a focal point in this blank region as well.
Considering these problems, even with the conventional diffractive multi-focal lens, items with a further increase in the number of focal points have been proposed. As specific examples, proposed previously by this patent applicant, there are Japanese Unexamined Patent Publication No. JP-A-2010-158315 (Patent Document 1) and PCT Japanese Translation Patent Publication No. JP-A-2013-517822 (Patent Document 2) showing the subordinate concepts thereof.